(i) Field of the Invention
This invention relates to a process, a system and an apparatus for the separation of a mixture of gases and vapors into its constituent components. One specific adaptation of this invention relates to the separation and recovery of oxygen from air. Another specific adaptation of this invention relates to the separation and recovery of hydrogen from a hydrocarbon gas mixture.
(ii) Description of the Prior Art
Many processes, systems and apparatuses have been proposed to separate a mixture of gases and vapors into its constituent components. Most involve either complicated procedures or require the use of equipment which is expensive both to assemble and to operate.
In one conventional type of gas-solid or vapor-solid adsorption process, the feed mixture is permitted to enter one end of a packed bed of adsorbent and the desired product is recovered from the opposite end. This process continues for a sufficient period of time, determined by the time when the bed becomes saturated with the more strongly adsorbed components and the product purity begins to deteriorate below acceptable limits. At this point regeneration of the adsorbent is accomplished by reducing the pressure and/or increasing the temperature of the adsorbent and by withdrawing the evolved contaminants from one end or other of the adsorbent bed. The resistance to flow offered by the adsorbent bed over its full length causes the elimination of the contaminants to occur very slowly. The time required for regeneration is often much longer than the time required for adsorption which is usually prohibitively long when a high frequency production-regeneration cycle is desired.
It has been discovered that, when a mixture of two or more gases or vapors is permitted to pass through a cylindrical column packed with an adsorbent which possesses a high affinity toward at least one of the components in the mixture, for some significant period of time, the effluent from the column is relatively free of the more strongly adsorbed components. Moreover, saturation of the adsorbent by these components proceeds in a rather peculiar fashion. A clearly defined interface is established between the saturated and unsaturated regions in the column, and this interface moves progressively through the column until complete saturation is achieved and the strongly adsorbed components suddenly emerge as contaminants in the effluent product. The concentration of these contaminants then quickly increases to the level obtaining in the feed mixture. Prior to this point, operation must be terminated and the adsorbent must be regenerated in preparation for another production cycle.
A thorough examination of the behaviour of adsorbers reveals that, regardless of the particular separation process involved, certain desirable features of geometry and mode of operation can be identified. Thus, it is most advantageous to provide:
1. An adsorbent which possesses a high capacity for the adsorbate (the strongly adsorbed component) to enhance both the quality and quantity of recovered effluent product.
2. A long time period before breakthrough of the contaminating component to increase the period of the cycle and permit sufficient time for regeneration of the saturated adsorber.
3. A minimum adsorption column cross-section to afford a uniform feed distribution and, hence, to minimize channelling of the contaminant through the column.
4. A high feed flow rate to improve the rate of mass transfer between the mixture and the adsorbent.
5. An adsorbent with a maximum tolerance for high concentrations of adsorbate in the feed.
6. A very small residual of the strongly adsorbed gas remaining on the adsorbent at the end of the regeneration portion of the cycle.
7. A rapid rate of regeneration of the adsorbent to minimize the non-productive portion of the cycle.
8. A geometry which permits a low pressure gradient across the adsorbent during regeneration in order to conserve energy.
While it is desirable to achieve these ends, this has not, in fact, been possible heretofore. It has not been posible, heretofore, to devise a simple geometry which permits the attainment of all of these objectives simultaneously. The first four, which pertain to the adsorption phase of the cycle, are favoured by a long column of minimum cross-section. The fifth is a function only of the properties of the adsorbent and gas mixture involved, and is unaffected by geometry. The sixth normally depends upon the period and type of regeneration and, in the case of vacuum regeneration, the residue decreases logarithmically with time. The last two, which pertain to the regeneration phase of the cycle, are improved by a reduction in length and an increase in cross-section of the column. Further, in this connection, it is noted that the time required for regeneration of a specified volume of adsorbent through application of a vacuum at either end of the adsorption column varies in a parabolic fashion with the length of the column. Clearly, regeneration of adsorbers of even modest length by this technique could require several hours.
One attempt to solve the problems noted above was suggested in Canadian Pat. No. 986,424 issued Mar. 30, 1976 to Robert A. Ritter and David G. Turnbull. In that patent, the improvement comprised passing the gas mixture, while under a positive pressure condition of up to 60 p.s.i.g. through a first adsorption zone containing an adsorbent material which is more selective to one gas than to another gas in that gaseous mixture, the adsorption zone also including a primary inlet means and a primary outlet means. The gas mixture was thus caused to travel a relatively long adsorption path from the inlet, through the adsorbent and out the primary outlet. By this means the gas which was more strongly adsorbed was retained in the adsorption zone and gas which was less strongly adsorbed by the adsorbent was withdrawn from the adsorption zone through a primary outlet zone which was substantially free of adsorbent material. The adsorbent was then regenerated and the more strongly adsorbed gas was removed from the adsorbent by the application of a subatmospheric pressure to the adsorbent in the adsorption zone through a distinct zone, a secondary outlet zone which was substantially free of adsorbent material. In this way, the desorbed gas travelled a relatively short, direct desorption path from the adsorbent to the distinct zone which was under subatmospheric pressure and then was removed through a secondary outlet connected to the subpressure distinct zone.
The above-identified Canadian Pat. No. 986,424 also provided an improved adsorption-desorption system for selectively separating one gas from a mixture of gases. The system comprised compressor means for subjecting the mixture of gases to superatmospheric pressure. First storage means were provided which were operatively connected to the compressor means for temporarily storing the mixture of gases under pressure. A first pair of adsorption-desorption vessels was provided which was operatively connected to the storage means by gas inlet lines. Each vessel included an adsorption zone, primary inlet means to the adsorption zone, primary outlet means from the adsorption zone, secondary inlet means to the adsorption zone, secondary outlet means from the adsorption zone, and valve means actuatable selectively to open one only of the primary inlet means and the primary outlet means, or the secondary inlet means and the secondary outlet means. Primary gas withdrawal lines were provided leading from the primary outlet means of each vessel of the first pair of adsorption-desorption vessels. Similarly, secondary gas discharge lines were provided leading from the secondary outlet means of each vessel of the first pair of adsorption-desorption vessels to a source of subatmospheric pressure. Finally, control means were provided for cyclically and alternately operating one adsorption-desorption vessel under adsorption conditions, where its primary inlet andoutlet were functional.
The above-identified Canadion Pat. No. 986,424 also provided an apparatus for separating one gas from at least one other gas in a mixture of gases. The apparatus included a main chamber which was adapted to contain adsorbent material. The chamber was provided with perforated walls (e.g., rigid perforated tubes) within the chamber and elastomeric diaphragms were associated with the perforated walls or the rigid perforated tubes. A primary inlet means was provided to the chamber and also a secondary inlet means was also provided, and alternative primary and secondary outlet means were provided from the main chamber. The primary inlet and outlet means were disposed at intervals throughout the adsorbent bed and were arranged such that the feed gas mixture, in moving from the inlet to the outlet, must travel a relatively long path through the adsorbent bed while the secondary outlet means was interconnected to the perforated walls, i.e., the perforated tube structures. Pressure means were provided which were selectively actuatable to urge the diaphragm into engagement with the perforated wall structure to provide an unperforated combined structure.
That above-identified Canadian Pat. No. 986,424 also provided a diaphragm valve. The valve included a hollow casing provided with main gas inlet and gas outlet ports. A hollow sleeve was disposed within the casing, this hollow sleeve including gas impermeable sealed ends and a central portion which was gas permeable or perforated or slotted. A pair of perforated members were disposed one at each end of the hollow sleeve member, and these members extended outwardly from the hollow sleeve member to the interior wall of the hollow casing. An elastomeric tubular diaphragm was secured to the ends of the sleeve and enveloped the central zone of the sleeve. The interior of the sleeve communicated to a source of a pressure fluid so that when the fluid pressure was applied, the diaphragm expanded to come into sealing engagement with the perforated members so as to provide a combined unperforated member. By these means, no gas flowed between the gas inlet and gas outlet ports. Releasing the pressure permitted gas flow between the inlet and outlet ports.
While the above-identified Canadian Pat. No. 986,424 was partially effective in solving some of the problems outlined above, certain other deficiencies were encountered due to the configuration in which the desorption elements comprised perforated tubes disposed within and extending the length of the adsorbent bed. The tubes enclosed full length diaphragm valves which, when expanded, closed the perforations and isolated the vacuum system from the bed. The presence of these long, relatively large diameter diaphragms in the desorption tubes represented a significant resistance to flow of desorbed adsorbate, especially at low absolute pressures, and, hence, tended to increase the pump down-time. Moreover, the frequent cyclical engagement of the diaphragm with the perforations in the tube wall caused excessive wear of the diaphragm and necessitated special treatment of the circumferential inner edge of the perforations. Finally, appreciable quantities of air were required to operate the large diaphragm. As a result, diaphragm expansion was slower and compressor capacity was needlessly consumed.
Other proposals for this type of gas separation and/or purification are taught in the following patents:
German Pat. No. 453,887, Dec. 20, 1927 PA1 German Pat. No. 578,485, June 14, 1933 PA1 U.S. Pat. No. 2,254,799, Sept. 2, 1941 PA1 U.S. Pat. No. 3,141,748, July 21, 1964 PA1 U.S. Pat. No. 3,242,651, Mar. 29, 1966 PA1 U.S. Pat. No. 3,430,418, Mar. 4, 1969 PA1 U.S. Pat. No. 3,533,221, Oct. 13, 1970 PA1 U.S. Pat. No. 3,619,984, Nov. 16, 1971 PA1 U.S. Pat. No. 3,719,025, Mar. 6, 1973 PA1 U.S. Pat. No. 3,923,477, Dec. 2, 1975 PA1 U.S. Pat. No. 3,957,463, May 18, 1976 PA1 U.S. Pat. No. 3,977,845, Aug. 31, 1976 PA1 U.S. Pat. No. 4,070,164, Jan. 24, 1978 PA1 U.S. Pat. No. 2,075,036, Mar. 1937 PA1 U.S. Pat. No. 3,080,219, Mar. 1963 PA1 U.S. Pat. No. 3,085,379, Apr. 1963 PA1 U.S. Pat. No. 3,140,931, July 1964 PA1 and U.S. Pat. No. 3,432,995, Mar. 1969.
In spite of the teachings of the above patents, the problem still exists of efficient adsorption and subsequent efficient desorption by significantly decreasing the time for regeneration of the adsorbent.